Laser detection system and method

ABSTRACT

A laser detection system comprises a plurality of lasers wherein each laser is configured to produce a respective laser beam for excitation of one or more different compounds, a sample cell for containing a volume of sample gas, at least one directing device configured to direct the laser beams to the sample cell, wherein the at least one directing device is configured to direct the laser beams along a common optical path to the sample cell, and a detector apparatus for detecting light output from the cell.

BACKGROUND

Continuous emission monitoring instruments are increasingly needed tomonitor industrial pollution output in various industrial sites, forexample at power plants, process industry factories and commercialshipping facilities. The need arises, for example, from efficiencyimprovements, health and safety considerations and legislativerequirements. It can be desirable to obtain measurements on a range ofemitted compounds, for example: sulphur dioxide, nitrogen oxides, carbonmonoxide, carbon dioxide, methane, water and oxygen.

Known gas analysis systems are sensitive to single compounds or a smallnumber of compounds. To cover multiple compounds using known systems itcan be necessary to install several different continuous emissionmonitoring instruments, which can be inefficient, complicated and takeup significant amount of space.

SUMMARY

In a first aspect of the invention, there is provided a laser detectionsystem comprising: a plurality of lasers wherein each laser isconfigured to produce a respective laser beam for excitation of one ormore different compounds; a sample cell for containing a volume ofsample gas; at least one directing device configured to direct the laserbeams to the sample cell, wherein the at least one directing device isconfigured to direct the laser beams along a common optical path to thesample cell, and a detector apparatus for detecting light output fromthe cell.

The at least one directing device may comprise a plurality of opticalcomponents arranged such that, for each laser beam a respective at leastone of the optical components is arranged to direct said laser beamalong the common optical path.

The plurality of optical components may be arranged substantially in astraight line.

The plurality of optical components may be arranged such that on thecommon optical path the laser beams may overlap by at least 90% of theirdiameters, optionally at least 50% of their diameters, optionally atleast 20% of their diameters, optionally at least 10% of theirdiameters. The laser beams may comprise infra-red light or visible lightor light of any other suitable wavelength or from any suitable part ofthe electromagnetic spectrum

Each of the lasers may be arranged such that in operation each of thelasers transmits its laser beam to its corresponding at least one of theoptical components in a direction substantially orthogonal to saidstraight line. At least one, optionally each, of the lasers may comprisea quantum cascade laser.

At least one of the optical components may comprise a flat or non-wedgedoptical component, optionally each of the optical components maycomprise a respective flat or non-wedged optical component.

The plurality of optical components may comprise at least one mirror,optionally at least one partially reflective mirror and/or at least onedichroic mirror.

Each of the optical components may have a thickness in a range 0.1 mm to1 mm.

The optical components may be arranged in series and may be configuredsuch that in operation each optical component directs a laser beam fromits associated laser to join said common optical path, and/or directs orallows passage of laser beam(s) from preceding optical components in theseries along said common optical path.

At least one, optionally each, of the optical components may be at leastpartially reflective and at least partially transmissive.

The at least one directing device may comprise steering optics betweenthe last of said plurality of optical components and the sample cell andconfigured to direct the laser beams into the optical cell.

The detection apparatus may comprise a plurality of detectors, eachdetector being configured to detect radiation of a respective wavelengthor range of wavelengths.

The system may further comprise:

-   -   a controller configured to control operation of the plurality of        lasers such that the laser beams are pulsed laser beams        interleaved in time.

The controller may be configured to synchronise operation of thedetection apparatus and the lasers, thereby to obtain a series ofdetection signals, each detection signal being associated with arespective one of the lasers.

The controller may be configured to control operation of the lasers suchthat each laser beam is pulsed at a rate in a range 1 kHz to 200 kHz,optionally in a range 10 kHz to 100 kHz, and/or wherein the controlleris configured to control the lasers such that each laser beams is pulsedwith pulse lengths in a range 100 ns to 5000 ns.

The system may further comprise a processing resource configured todetermine an amount of NOx based on the detected light outputs.

The plurality of compounds may comprise at least one of: NO, NO₂, H₂O,CO, CO₂, CH₄, SO₂, NH₃, C₂H₂, and O₂.

Each of the plurality of lasers may be configured to produce infraredlaser radiation.

Each of the lasers may be configured to produce a laser beam of arespective different wavelength or range of wavelengths.

At least one, optionally each, of the ranges of wavelengths may beselected from the following ranges: 5.2632 to 5.2356 μm; 6.1538 to6.1162 μm; 4.4742 to 4.4743 μm; 7.4627 to 7.4349 μm; 0.7605 to 0.7599μm; and 10.0 to 10.2 μm.

The detector apparatus may be arranged on the opposite side of thesample cell to the plurality of lasers and the at least one directingdevice.

The system may further comprise a gas supply arrangement configured tosupply sample, optionally a remote sample gas, to the sample cell.

The sample cell may comprise at least one of a Herriot cell, amulti-pass cell.

The system may be a continuous emission monitoring system.

In a further aspect of the invention, which may be providedindependently, there is provided a method of detecting a plurality ofdifferent compounds, comprising producing a plurality of laser beams,each for excitation of one or more different ones of the compounds,directing the laser beams along a common optical path to a sample cellfor containing a volume of sample gas, and detecting light output fromthe cell.

Features in one aspect may be applied as features in another aspect inany appropriate combination. For example, method features may be appliedas system features or vice versa.

BRIEF DESCRIPTION OF THE DRAWINGS

Various aspects of the invention will now be described by way of exampleonly and with reference to the accompanying drawings, of which:

FIG. 1 is a schematic representation of a laser spectroscopy system;

FIG. 2 is a schematic view of a laser module of the laser spectroscopysystem; and

FIG. 3 is a perspective view of housing for the laser spectroscopysystem;

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a laser spectroscopy system foranalysing gas collected in a sample cell 10 of a sensor apparatus 12.The system comprises a laser module 14 that is optically coupled to thesensor apparatus 12. The system also includes a controller 16 that iselectronically, electrically or otherwise connected to the laser module14 and the sensor apparatus 12. The laser module 14 comprises aplurality of lasers 18 and at least one directing device in the form ofa plurality of optical components 20 arranged to direct laser beams fromthe lasers along a common optical path into the sample cell 10, asdescribed in more detail below in relation to FIG. 2.

In addition to the sample cell 10, the sensor apparatus 12 also includessteering optical components 22 and a detector apparatus 24 comprising aplurality of detectors. The detectors are configured to detect lightfrom the sample cell. The light may be infra-red or visible light orlight of any other suitable wavelength or from any suitable part of theelectromagnetic spectrum. The controller 16 comprises a control module26 and a signal processor 28. The control module 26 is configured tocontrol operation of the lasers and the signal processor 28 isconfigured to process signals obtained from the detector apparatus 24.The controller 16 may be, for example, in the form of a suitablyprogrammed PC or other computer, or may comprise dedicated circuitry orother hardware, for example one or more ASICs or FPGAs or any suitablemixture of hardware and software. The control module 26 and processingmodule may be provided as separate, distinct components in someembodiments, for example separate processing resources, rather thanbeing provided within the same controller component as shown in FIG. 1.

The sample cell 10 has an optical entrance aperture and an optical exitaperture. The sample cell 10 may, for example, be a Herriot cell or anyother suitable type of sample cell. The sample cell 10 of FIG. 1 definesa volume into which a sample of gas can be introduced and collected. Thegas can comprise one or more different compounds of interest. Anindication of the presence of these compounds in the gas collected inthe sample cell 10 can be determined by passing light from the lasers 18through the sample cell 10. If the light is in a wavelength range thatcorresponds to the absorption spectrum or absorption lines of thecompound of interest, then any absorption of light as it passes throughthe cell may be due to the presence of the compound of interest in thesample. The level of absorption, once determined, can be used todetermine a physical property of the compound of interest in the sample,for example, concentration. As different compounds have absorptionspectra at different wavelength, different wavelengths of light areprovided to the sample cell 10.

FIG. 2 is a more detailed schematic view of a part of the laser module14 of the laser spectroscopy system shown in FIG. 1. The opticalcomponents 20 comprise a set of partially reflective mirrors 32 and adichroic mirror 34. The partially reflective mirrors 32 comprise a firstmirror 36, a second mirror 38, a third mirror 40, a fourth mirror 42 anda fifth mirror 44. The lasers 18 comprise a first laser 46, a secondlaser 48, a third laser 50, a fourth laser 52, a fifth laser 54 and asixth laser 56. The partially reflective mirrors 32 and the dichroicmirror 34 are configured to direct laser beams from the lasers 18 alonga common optical path to point 58. Additional steering opticalcomponents to steer the combined laser beam 30 from point 58 along thecommon optical path to the sample cell 10 are included in the system butnot shown in FIG. 2. Each of the lasers 46, 48, 50, 52, 54, 56 has acorresponding mirror 36, 38, 40, 42, 44, 34. The partially reflectivemirrors 32 and the dichroic mirror 34 are arranged in a straight line.Each mirror is tilted with respect to this straight line at a 45 degreeangle. The straight line defines a direction of propagation from thefirst mirror 36 to the dichroic mirror 34 and then to point 58. Acombined laser beam 30 propagates along the direction of propagation.

Any suitable partially reflective mirrors may be used. In the embodimentof FIG. 2, each of the partially reflective mirrors comprise coatedinfrared BaF₂ or CaF₂ windows that have an optical coating applied tocontrol broadband reflection of the front surface. Any other suitablematerials can be used in alternative embodiments. In the embodiment ofFIG. 2, two coatings are used, an 80:20 (80% transmission, 20%reflection) and a 50:50 (50% transmission, 50% reflection). This canallow the variety of laser powers to be adjusted to harmonise the outputpower to a consistent value for each laser (within practical limits).More or fewer coatings can be used in alternative embodiments. Thecoatings of the partially reflective mirrors of FIG. 2 are designed tobe broadband, such that any variation in their response to a change inwavelength, particularly around wavelengths of interest, is reduced orminimised.

Any suitable dichroic mirrors may be used. In the embodiment of FIG. 2,the dichroic mirror comprises a coated infrared BaF2 window that has anoptical coating applied to cause light lower than a specified wavelengthto be reflected and light higher than said specified wavelength to betransmitted. Any other suitable materials can be used in alternativeembodiments. In the embodiment of FIG. 2, the coating is such as toreflect light less than 1 μm in wavelength and to transmit light greaterthan 1 μm in wavelength.

In other embodiments, other suitable types of mirror or optical devicesmay be used in place of the partially reflective mirrors and thedichroic mirror. For example, in some embodiments a mirror other than adichroic mirror or partially reflective mirror may be used at theposition of dichroic mirror 34, e.g. at the last mirror position beforepoint 58. Such a mirror may be used at the last position to introducemore power into the cell. This can be possible as or if the lastposition does not have any additional lasers behind it such that nolasers need to pass through the last position. In alternativeembodiments, any suitable number and combination of partially reflectivemirrors and dichroic mirrors may be used.

Each of the partially reflective mirrors 32 is configured to partiallyreflect and partially transmit light incident on it. The reflection andtransmission properties of the mirror are chosen to direct laser beamsfrom the lasers 18 along the common optical path. In the embodiment ofFIG. 2, each of the partially reflective mirrors 32 reflects 20% of theincident light and transmits 80% of the incident light from thecorresponding one of the lasers 18. The partially reflective mirrors 32may have different reflection and transmission properties in alternativeembodiments. The dichroic mirror 34 is defined by a reflectionwavelength range and is configured to reflect light that has awavelength in the reflection wavelength range and transmit light with awavelength outside the reflection wavelength range. The reflectionwavelength range of the dichroic mirror 34 is chosen to correspond to awavelength range of the sixth laser 56, such that light from the sixthlaser 56 is reflected and light from the first to fifth lasers istransmitted. The mirrors are flat or non-wedged optical components.Advantageously this allows the system to operate in an orthogonalfashion. For example, the system has a geometrical arrangement such thatthe direction of propagation from the first mirror 36 to the dichroicmirror 34 is substantially orthogonal to the laser beams output from thelasers 18.

Another advantage of using flat or non-wedged optical components inembodiments is that the directing of the laser beams to the commonoptical path may be substantially independent of wavelength, for examplesuch that any distortion effects or other artefacts caused by theoptical components may be substantially independent of wavelength.However, the use of mirrors may cause the resulting optical signal to besubject to fringe interference effects. These effects can be reduced byselecting the dimensions, in particular the thickness, of the opticalcomponents to control the Free Spectral Range of the system.

The Free Spectral Range is a measure of the wavelength differencebetween two successive maxima or minima. Typically, a suitable thicknessof the optical components is less than 1 mm. This choice presents atworse a Free Spectral Range of 4cm⁻¹ or greater. By controlling the FreeSpectral Range, the frequency at which fringing effects occur can beshifted to not coincide and/or interfere with the measurement of thecompounds in the sample cell 10.

The Free Spectral Range of this magnitude provides a spectral windowthat is similar in width to the spectral window covered by an entirelaser scan. An expected effect is a curvature on the background of thelaser pulse. This background can be easily removed using spectralfitting algorithms as part of the processing the signal. Additionalfringing effects are avoided in the steering optical components 22 inthe sensor apparatus 12 and optics used to steer light to the samplecell 10 through the use of non-flat or wedged optical components.

Each laser in FIG. 2 has a corresponding mirror belonging to the set offive partially reflective mirrors 32 and one dichroic mirror 34. Inoperation a laser beam from the first laser 46 passes to the firstmirror 36 and then from the first mirror 36 to the point 58. The firstmirror 36 is tilted such that the laser beam from the first laser 46 isreflected at a right angle by the first mirror 36. Likewise, each of thesecond to fifth lasers has a corresponding optical path defined by thesecond to fifth mirrors. A sixth optical path is defined in the same wayfrom the sixth laser 56 to the dichroic mirror 34 and to the point 58.All of the mirrors are arranged at the same tilted angle as the firstmirror 36 such that each of the optical paths bends at a right angle atits point of intersection with its corresponding mirror.

The mirrors are arranged such that laser beams from the lasers 46, 48,50, 52, 54, 56 pass along a common optical path to the cell 10 via point58 after reflection by their corresponding optical components 36, 38,40, 42, 44, 34. The common optical path may, for example, have one endat the first mirror 36 and the other end at the entrance aperture 84 tothe sample cell 10 and may extend through point 58 and when directed topass along the common optical path, the optical paths of each respectivelaser joins the common optical path. Hence, the optical paths of eachlaser may substantially overlap.

In operation, the lasers 18 are controlled by the control module 26, orother control component in other embodiments, to sequentially producepulses. The sequence may be as follows. The first laser 46 produces afirst pulse that is directed to point 58 by the optical components andpasses onward to the sample cell 10. Subsequently, the second laser 48produces a second pulse that is directed to point 58 by the opticalcomponents and passes onward to the sample cell 10. This is followed, inturn, by a third pulse produced by the third laser 50 that is directedto point 58 by the optical components and passes onward to the samplecell 10, a fourth pulse produced by the fourth laser 52 that is directedto point 58 by the optical components and passes onward to the samplecell 10, a fifth pulse produced by the fifth laser 54 that is directedto point 58 by the optical components and passes onward to the samplecell 10, and a sixth pulse produced by the sixth laser 56 that isdirected to point 58 by the optical components and passes onward to thesample cell 10. Following the sixth pulse, this sequence is repeated.The pulsed beams from each of the lasers are interleaved and/ornon-overlapping in time and propagate along the common path to thesample cell 10

Following the above sequence, the first pulse is incident on, andreflected by, the first mirror 36 and is then transmitted by the second,third, fourth, fifth and dichroic mirror 34 to point 58 and continues tothe sample cell 10 and the detector apparatus 24. Subsequently, thesecond pulse is incident on, and reflected by, the second mirror 38 andis then transmitted by the third, fourth, fifth and dichroic mirror 34to point 58 and onward to the sample cell 10 and detector apparatus 24.Subsequently, the third pulse is incident on, and reflected by, thethird mirror 40 and then transmitted by the fourth, fifth and dichroicmirror 34 to point 58 and onward to the sample cell 10 and detectorapparatus 24. Subsequently, the fourth pulse is incident on, andreflected by, the fourth mirror 42 and is then transmitted by the fifthmirror 44 and the dichroic mirror 34 to point 58 and onward to thesample cell 10 and detector apparatus 24. Subsequently, the fifth pulseis incident on, and reflected by, fifth mirror 44 and is thentransmitted by the dichroic mirror 34 to point 58 and onward to thesample cell 10 and detector apparatus 24. The last pulse in the sequenceis the sixth pulse and this pulse is incident on and reflected by thedichroic mirror 34 to point 58 and onward to the sample cell 10 anddetector apparatus 24. The pulse sequence is then repeated.

The pulses propagate through the sample cell 10 to the sensor apparatus12. The steering optical components 22 in the sensor apparatus 12 steerlight (originating from the first to fifth lasers) from the cell to afirst detector that is sensitive to light from the first to fifthlasers. Thus, in this embodiment one of the detectors is sensitive tolight from more than one of the lasers. The steering optical components22 in the sensor apparatus 12 steer light (originating from the sixthlaser) from the cell to a second detector that is sensitive to lightfrom the sixth laser 56. The steering optical components 22 include asecond dichroic mirror to direct light of the sixth laser 56 towards thesecond detector and to direct light of the first to fifth lasers to thefirst detector. The optical properties of the second dichroic mirror maymatch the properties of the dichroic mirror 34 of the laser module 14.The steering optical components 22 include two separate off-axisparabolic mirrors to focus the two different branches of light onto thetwo detectors. The control module synchronises operation of the lasersand the first and second detectors, such that each of the detectionsignals corresponds to light received from a respective one of thelasers.

The lasers 18 of FIG. 1 are semiconductor diode lasers that are operableto produce light over a sub-range of wavelengths. The lasers may bequantum cascade lasers, for example pulsed, chirped quantum cascadelasers, although any other suitable types of laser may be used inalternative embodiments. The lasers may, for example, produce beams of 2to 3 mm in diameter, or of any other suitable size.

The sub-ranges of wavelengths may be in the infra-red spectrum. Thewavelength ranges are chosen to correspond to the measurement of one ormore compounds. Together the instrument may provide multiple wavelengthranges of light and combines, for example, visible, near infrared and/ormid infrared light to take advantage of the most suitable wavelengthsfor each compound. Table 1 shows an example implementation of wavelengthranges for lasers 18, the corresponding wavenumber range and thecorresponding compound detected by light in this wavelength range:

TABLE 1 Wavelength Wavenumber Laser Range (μm) Range (cm⁻¹) CompoundsDetected 1 5.2632-5.2356 1900-1910 Nitrogen Oxide (NO), Water (H2O) 26.1538-6.1162 1625-1635 Nitrogen Dioxide (NO2) 3 4.4742-4.4743 2225-2235Carbon Monoxide (CO), Carbon Dioxide (CO2) 4 7.4627-7.4349 1340-1345Methane (CH4), Sulphur Dioxide (SO2) 5 10.0-10.2  980-1000 Ammonia(NH3), Acetylene (C2H2) 6 0.7605-0.7599 13150-13160 Oxygen (O2)

Careful selection of wavelength ranges of the lasers allows multiplemeasurements per laser wavelength. As can be seen in Table 1, thewavelength ranges of the first five lasers are of the same order ofmagnitude. However, the wavelength range of the sixth laser to detectOxygen is an order of magnitude smaller. The first and second detectorsare selected to detect light in the wavelength ranges of the first tofifth laser, or the wavelength range of the sixth detector respectively.

The control module 26 is configured to send one or more electroniccontrol signals to the lasers 18. In response to the electronic controlsignals, the lasers 18 produce the combined laser beam 30. The controlsignal acts to pulse the lasers 18 sequentially. In other words, thecontrol signal acts to drive each of the lasers 18 in a sequence, suchthat over a sample time interval only light from one laser is providedto the optical components 20. The optical components 20 are configuredto direct the light from each laser along the optical path of the laserto follow the common path to the sample cell 10. In this way, thecontrol module 26 controls the laser module 14 to produce the combinedlaser beam 30 and provide the combined laser beam 30 to the sample cell10.

The switching frequency between the lasers is selected to ensure areliable measurement in the sensor apparatus 12. In particular, the timetaken for a pulse of light to traverse its sample cell optical path isdependent on the physical properties of the pulse and the dimensions ofthe sample cell 10. If light from more than one laser is incident in thesample cell 10 over a sample time interval then interference can occurleading to an unreliable measurement. Therefore, the pulse lengths andfrequency of subsequent laser pulses are controlled and selected to takeinto account the time taken by light to traverse its sample cell opticalpath to ensure that light from only one laser is present inside thesample cell 10 over a sample time interval. Suitable pulse durations forpulses from the lasers 18 may be between 100 nanoseconds and 5000nanoseconds. The frequency of sequential pulsing may be up to 100 kHz insome embodiments.

The signal processor 28 processes the detection signals from thedetectors to determine the concentrations and/or relative amounts of thedifferent compounds under investigation, or to determine any otherdesired properties. The signal processor 28 uses any suitable knownprocessing techniques to determine the concentrations, relative amountsor other properties.

A calibration mechanism may also be provided. An example calibrationmechanism comprises a camera and a mirror adjustment mechanism. Thecamera or is positioned at or near the point 58 to intersect a desireddirection of propagation of the combined laser beam 30. The desireddirection of propagation is such that the combined laser beam 30 will,in normal operation, enter the sample cell 10 via the common opticalpath. During a calibration step, sample beams are produced by the lasers18 and the sample beams are directed by the optical components 20 to thecamera. The camera detects the position of the sample beams incident onit relative to the desired direction of propagation. The mirroradjustment mechanism adjusts the position, in particular the tiltrelative to the direction of propagation, of the partially reflectivemirrors 32 and dichroic mirror 34 to substantially align the opticalpaths of the lasers 18 with the desired direction of propagation andsubstantially align the optical paths with each other. For example, theoptical paths are substantially aligned within a 1.1° tolerance. Thecalibration step is repeated for each of the lasers 18.

FIG. 3 is a perspective view of housing for the laser spectroscopysystem. The housing has an upper section 60 and a lower section 62. Theupper section 60 has a lift-off cover 64 that is secured in a closedposition by a first and second release catch 66. The sample cell 10 islocated in the upper section 60 of the housing. A gas supply arrangementin the form of sample supply tube 68 provides gas to the sample cell. Asample return tube 70 provides an outlet for gas from the sample cell.Ventilation is provided to the sample cell via a vent 72 in the uppersection 60. The lower section 62 has a local operator user input display74 and a purge control display 76. In the embodiment of FIG. 3, the userinput display is for interaction with the analyser and visualcommunication of measurements and status. Some maintenance functionalityis provided by the user input display in this embodiment, however itspurpose is mostly communication of measurement values and status.

The purge control display 76 of the embodiment of FIG. 3 is used controlthe air purge of the enclosure. This can be a requirement of hazardousarea installations where steps must be taken to prevent fire hazards. Inthis case an air purge controlled via the purge control display 76supplies, for example constantly supplies, the enclosure or housing ofthe system with clean air to prevent an explosive environment frombuilding up.

Also connected to the lower section 62 are three output conduits 78. Theconduits provide electrical breakthroughs that allow power and controlsignals to be sent to the system and to allow data to be transmittedfrom the system. The data transmitted may, for example, be in the formof digital signals, digital health signals, analogue signals for example4-20 mA signals indicating measured values of gases, more sophisticatedprotocols such as Modbus, or in any other suitable format. Thearrangement described above provides a compact system. In someembodiments, the housing may be around 550 cm long, the upper sectionmay be around 200 cm tall and the lower section may be around 370 cmtall.

The sample supply tube 68 and the sample return tube 70 provides a fluidcommunication path through the sample cell. The sample gas can becollected from a remote location and can be delivered via the samplesupply tube 68 to the sample cell to be sampled. The sample gas can thenbe exhausted from the sample cell via the sample return tube 70.Together, the sample supply tube 68 and the sample return tube 70 allowfor the instrument to operate remotely, in contrast to in-situ emissionsensing. Any other suitable gas supply arrangement may be used inalternative embodiments.

A sample handling system (SHS) unit (not shown) may be provided tocontrol pressure of the gas in the sample cell 10. Any suitable SHS unitor other pressure control device may be used, which may or may notcomprise or be driven by a pump and may or may not comprise otherpressure control components such as an arrangement of valves. In theembodiment of FIG. 3, the SHS unit includes an aspirator rather than apump, although a pump or other pressure control device or components maybe used in other embodiments.

Additionally, the housing contains at least one absorber component toabsorb laser light that is not directed along the common path to thesample cell 10. The at least one absorber component may containadditional optical components, for example wedged optical components.

Any suitable sample cell may be used as sample cell 10. In theembodiment of FIGS. 1 to 3, a Herriot cell is used as the sample cell.Any suitable Herriot cell may be used, or any suitable multipassspectroscopic absorption cell, or for example any other cell which isconfigured to provide interaction between the laser beam(s) and thesample gas, for instance by way of reflection of the laser beam betweensurfaces of a chamber containing the gas.

A skilled person will appreciate that variations of the describedembodiments are possible without departing from the scope of the claimedinvention. For example, while it is discussed that a control module inthe controller is used to sequentially pulse the output of the lasersallowing the combined beam to be produced, other controller arrangementscan also be used. One alternative is a mechanical optical switchingarrangement that physically controls laser light such that only onelaser provides light to the optical components over a given interval oftime. As another example, the lasers described are semiconductor diodelasers that operate over a wavelength range. However, the lasers may beany suitable radiation source capable of providing suitable wavelengthsof light. Additionally, the lasers may be single wavelength. Anotherexample, of a modification is to replace the off-axis parabolic mirrorswith any suitable focussing arrangement. Accordingly, the abovedescription of the specific embodiments is made by way of example onlyand not for the purposes of limitations. It will be clear to the skilledperson that minor modifications may be made without significant changesto the operations described.

1. A laser detection system comprising: a plurality of lasers whereineach laser is configured to produce a respective laser beam forexcitation of one or more different compounds; a sample cell forcontaining a volume of sample gas; at least one directing deviceconfigured to direct the laser beams to the sample cell, wherein the atleast one directing device is configured to direct the laser beams alonga common optical path to the sample cell, and a detector apparatus fordetecting light output from the cell.
 2. A system according to claim 1,wherein the at least one directing device comprises a plurality ofoptical components arranged such that, for each laser beam a respectiveat least one of the optical components is arranged to direct said laserbeam along the common optical path.
 3. A system according to claim 2,wherein the plurality of optical components are arranged substantiallyin a straight line.
 4. A system according to claim 2, wherein each ofthe lasers is arranged such that in operation each of the laserstransmits its laser beam to its corresponding at least one of theoptical components in a direction substantially orthogonal to saidstraight line.
 5. A system according to claim 2, wherein at least one ofthe optical components comprises a flat or non-wedged optical component6. A system according to claim 2, where the plurality of opticalcomponents comprise at least one partially reflective mirror and/or atleast one dichroic mirror.
 7. A system according to claim 2, whereineach of the optical components has a thickness in a range 0.1 mm to 1mm.
 8. A system according to claim 2, wherein the optical components arearranged in series and are configured such that in operation eachoptical component directs a laser beam from its associated laser to joinsaid common optical path, and directs or allows passage of laser beam(s)from preceding optical components in the series along said commonoptical path.
 9. A system according to claim 2, wherein each of theoptical components is at least partially reflective and at leastpartially transmissive.
 10. A system according to claim 2, wherein theat least one directing device comprises steering optics between the lastof said plurality of optical components and the sample cell andconfigured to direct the laser beams into the optical cell.
 11. A systemaccording to claim 1, wherein the detection apparatus comprises aplurality of detectors, each detector being configured to detectradiation of a respective wavelength or range of wavelengths.
 12. Asystem according to claim 1, further comprising: a controller configuredto control operation of the plurality of lasers such that the laserbeams are pulsed laser beams interleaved in time.
 13. A system accordingto claim 12, wherein the controller is configured to synchroniseoperation of the detection apparatus and the lasers, thereby to obtain aseries of detection signals, each detection signal being associated witha respective one of the lasers.
 14. A system according to claim 12,wherein the controller is configured to control operation of the laserssuch that each laser beams is pulsed at a rate in a range 1 kHz to 200kHz, optionally in a range 10 kHz to 100 kHz, and/or wherein thecontroller is configured to control the lasers such that each laserbeams is pulsed with pulse lengths in a range 100 ns to 5000 ns.
 15. Asystem according to claim 1, further comprising a processing resourceconfigured to determine an amount of NOx based on the detected lightoutputs.
 16. A system according to claim 1, wherein the plurality ofcompounds comprise at least one of: NO, NO₂, H₂O, CO, CO₂, CH₄, SO₂,NH₃, O₂H₂ and O₂.
 17. A system according to claim 1, wherein each of theplurality of lasers is configured to produce infrared laser radiation.18. A system according to claim 1, wherein each of the lasers isconfigured to produce a laser beam of a respective different wavelengthor range of wavelengths.
 19. A system according to claim 18, wherein atleast one of the ranges of wavelengths is selected from the followingranges: 5.2632 to 5.2356 μm; 6.1538 to 6.1162 μm; 4.4742 to 4.4743 μm;7.4627 to 7.4349 μm; 0.7605 to 0.7599 μm; and 10.0 to 10.2 μm.
 20. Asystem according to claim 1, wherein the detector apparatus is arrangedon the opposite side of the sample cell to the plurality of lasers andthe at least one directing device.
 21. A system according to claim 1,further comprising a gas supply arrangement configured to supply aremote sample gas to the sample cell.
 22. A system according to claim 1,wherein the sample cell comprises a Herriot cell.
 23. A system accordingto claim 1, wherein the system is a continuous emission monitoringsystem.
 24. A method of detecting a plurality of different compounds,comprising producing a plurality of laser beams, each for excitation ofone or more different ones of the compounds, directing the laser beamsalong a common optical path to a sample cell for containing a volume ofsample gas, and detecting light output from the cell.